J-COUPLING IN CHEMICALLY EQUIVALENT SPIN PAIRS AS STUDIED BY SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE

The presence of J coupling was observed in the solid-state nuclear magnetic resonance (NMR) spectrum of the P-31 spin pair in polycrystalline 1,2-bis (2,4,6-tri-tert-butylphenyl)-diphosphene (TBPDP) using the magic-angle spinning (MAS) technique. This represents the first example of J coupling in a spin-pair system with nuclei having identical isotropic chemical shifts. Average Hamiltonian theory is used to derive the time-independent eigenstates for the system. It is shown that a second-order perturbation term, which is dependent on the difference between the chemical-shift tensor orientations in addition to the strength of the dipolar coupling, allows the recoupling of the J interaction for the two chemically equivalent P-31 nuclei. In the absence of the perturbation term, the system is reduced to an A2 case in solution-state NMR for which, of course, no J coupling may be observed. The magnitude of the J coupling, namely, 580 +/- 20 Hz obtained from two-dimensional (2D) J-resolved experiments was found to be in excellent agreement with the value of 577 +/- 15 Hz estimated using the abovementioned theory and experimental data from the one-dimensional (1D) spectra measured as a function of the rotor spinning frequency.